At present, as fluid flow sensors that are installed in electronic control fuel injection apparatuses of internal combustion engines of vehicles, etc. and are used for air flow meters for measuring intake air quantity, the thermal type sensor has become mainstream because of its capability of directly detecting mass air quantity.
Among these sensors, especially a thermal air flow sensor manufactured by a semiconductor micromachining technology has attracted attentions because it can reduce a cost and is drivable with a low electric power. There are the following documents as conventional technologies of air flow sensors like this. For example, Japanese Unexamined Patent Application Publication No. S60 (1985)-142268 discloses an air flow sensor in which the heating resistor and a temperature measuring resistor (sensor) for measuring air flow rate are placed on a cavity formed by anisotropically etching a part of a Si substrate. Here, in Japanese Unexamined Patent Application Publication No. S60(1985)-142268, circuits of heater heating control, compensation of the flow rate output, and the like are produced on a different substrate. Therefore, the air flow rate detecting substrate and the circuit substrate need to be wire-connected by wire bonding, which gives problems of an increased cost caused by an increase in the number of parts, an increase in the number of processes by additional inspection after each assembly, lowering in a yield by assembly failure, etc.
There is Japanese Unexamined Patent Application Publication No. 2008-157742 as a technology for addressing these problems. This document discloses a thermal air flow meter in which a circuit part including MOS transistors and diode elements and a flow rate detection part having a diaphragm made by removing the Si substrate are formed on the same substrate. Therefore, a hard wiring process, such as the above-mentioned wire bonding, becomes unnecessary, and components can be reduced in count.